Metastable austenite stainless steel strip or steel sheet and manufacturing method thereof

ABSTRACT

A metastable austenite stainless steel strip of high strength and ductility. The steel strip including C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe, a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ T  phase and a γ R  phase, a sum of the γ T  phase and the γ R  phase is 15 to 50 volume %, and a γ T  phase area ratio defined by formula (2) (=100×(total area ratio of γ T  phase of entire observation area)) is between 1% and 20% inclusive, and 0.2% yield strength (YS) of 1400 N/mm 2  to 1900 N/mm 2  where a value of YS-EL balance satisfies at least 21,000 to 48,000.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of PCT Application No. PCT/JP2017/020147, filed May 30, 2017 and based upon and claiming the benefit of priority from prior Japanese Patent Applications No. 2016-109695, filed Jun. 1, 2016; and No. 2016-225085, filed Nov. 18, 2016, the entire contents of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a metastable austenite stainless steel strip or steel sheet having well-γ balanced strength and ductility and a manufacturing method of the same.

Description of the Related Art

Functioning members of devices such as smartphones, clamshell computers, and cameras and highly durable frame structure members of automobiles and airplanes are required to be thinned and lightened through a reinforcement process while their workability and size accuracy are maintained satisfactorily. Furthermore, when the devices and machines are miniaturized, a workload of these members increases, and thus, an excellent durability is required to be used repeatedly in a harsh environment.

Especially, in the technical field of the frame structure members for automobiles, members of high strength and high ductility have been developed for a long time. For example, a γ-SUS steel and a twinning induced plasticity (TWIP) steel which contain more than 20 mass % of Mn and/or Ni have been developed. Such steels have a conventional strength to ductility balance of a transformation induced plasticity (TRIP) steel. However, such a high strength and high ductility steel requires expensive cost of additional elements and has difficulty to be subjected to a cold rolling process to manufacture a steel strip and a steel sheet. Furthermore, since many conventional steels do not contain Cr, corrosion resistance is insufficient, and thus, an antirust process is required.

With a low alloy TRIP dual phase steel which is a center of attention recently, a result of TS:980 MPa-EL:30% and TS:1180 MPa-EL:25% is obtained (cf. Non Patent Literature 1). However, even the steel with the above property is insufficient. A steel strip or a steel sheet having both a yield strength (YP) of 1400 MPa or more which is required as a structural member and a high ductility has not been realized yet.

For example, Patent Literature 1 (JP2002-173742 A) disclose, as a method to improve flatness of steel, a manufacturing method of a high strength austenite stainless steel strip with a good flatness, having a Vickers hardness of 400 or more by performing a solution treatment of a stainless steel, then generating a strain induced transformation martensite phase (α′ phase) by cold rolling, and performing a reverse transformation process which generates a γ_(T) phase (reverse transformation austenite phase) of 3 volume % or more in a α′ phase.

However, the amount of γ_(T) phase greatly depends on a temperature, and although it may vary with chemical components, the amount of γ_(T) phase exceeds approximately 60% through a reverse transformation process in a temperature of 500° C. or more, and a strength of 1400 N/mm² or more is difficult to achieve. Furthermore, when the reverse transformation process is performed for a short period of time (for example, one to five minutes), ductility is improved to some extent, and when the reverse transformation process is performed for a longer period of time (for example, five to fifteen minutes), the ductility decreases rapidly. That is, the reverse transformation process is a very unstable process, and a steel strip or a steel sheet with a stable mechanical characteristics is difficult to achieve. Furthermore, a gain of 0.2% yield strength is few since carbide precipitation of Cr—C and Mo—C does not progress well. Therefore, with the manufacturing method of Patent Literature 1, a steel of both high strength and high ductility is practically impossible.

Patent Literature 2: JP54-120223 A discloses a stainless steel containing components similar to those of the stainless steel strip or steel sheet of the present invention, and it discloses a solution treatment and low temperature annealing in a temperature of 400° C. However, Patent Literature 2 discloses adding 2.0% or less of Mo (in its specification, only 1.15% of Example 9) as an effective element to increase the corrosion resistance, and Mo is not added as a precipitation strengthening component. Furthermore, with such a small amount of Mo, a precipitation strengthening function in a low temperature heat treatment is difficult to achieve.

Patent Literature 3: JP2012-201924 A discloses a stainless steel sheet manufactured through annealing in a temperature of 700 to 1100° C., cold rolling of 10% or more, and aging treatment in 300° C. However, the stainless steel sheet does not contain Mo and a precipitation strengthening function in a low temperature heat treatment with addition of Mo is not achieved.

Furthermore, Non-Patent Literature 2 discloses a target steel which aims a balanced tensile strength (TS) and elongation (EL) relationship within a range of 300 to 500° C., and therein, the tensile strength (TS) increases to approximately 1750 N/mm² while 0.2% yield strength is approximately 1250 N/mm². Furthermore, the target steel of Non-Patent Literature 2 is a Fe—Cr—C steel with a γ phase as its parent phase, and it is out of the category of metastable austenite stainless steels as in the present invention.

As conventional stainless steels containing Cr of 12 mass % or more, there are metastable austenite stainless steels such as SUS304 and SUS301. SUS301 is a steel which can reduce Ni content to perform strain induced transformation from an austenite (γ phase) to a martensite (α′ phase) through cold rolling in order to acquire more strength. These types of stainless steels are advantageous in individual characteristic such as strength and workability; however, to achieve 0.2% yield strength (YS) over 1400 N/mm², the elongation (EL) becomes 10% or less and a YS-EL balance (an indexing value by YS×EL) is approximately 14000. Thus, as a member used in a miniaturized and complex device, such a steel cannot exert a sufficient balance of strength and ductility and a reliability as a product cannot be sufficient.

In order to increase the strength after the formation of the member, there is a SUS631 precipitation hardening stainless steel which uses the chemical components of the SUS301 with approximately 1% of Al added thereto and is formed through precipitation strengthening by Ni₃Al. To manufacture this type of steel, a precipitation hardening heat treatment is required after the casting, and thus, a secondary processing manufacturer must bare increased costs, and strain and unevenness in size may be caused by the heat treatment. Furthermore, the ductility of the member decreases by precipitation hardening and thus, the toughness of the member itself decreases. In consideration of the above, users have demanded a material having well balanced strength and ductility which does not require an after treatment (such as heat treatment which may cause a size change) in the manufacturing process.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2002-173742 A -   Patent Literature 2: JP S54-120223 A -   Patent Literature 3: JP 2012-201924 A

Non-Patent Literature

-   Non-Patent Literature 1: Iron and Steel, Vol. 100 (2014) No. 1, P.     82-93 -   Non-Patent Literature 2: Nanoscale austenite reversion through     partitioning, segregation and kinetic freezing: Example of a ductile     2 GPa Fe—Cr—C steel, L. Yuan et al., Acta Materialia 60 (2012), p.     2790-2804.

BRIEF SUMMARY OF THE INVENTION Problem to be Solved by Invention

Inventors of the present application focused on a potential of an α′ phase generated by strain induced transformation and tried to raise 0.2% yield strength (YS) of a metastable austenite stainless steel to approximately 1400 N/mm².

Conventional metastable austenite stainless steels can achieve a ductility in a pre-treatment stage and a strength in a post-treatment stage by strain induced transformation through cold rolling and aging precipitation strengthening whereas there occur problems such as high costs of the aging precipitation strengthening and size change. Especially, if a size change occurs in electronic components and devices which must be designed finely, performance of an end product is greatly harmed, and thus, a heat treatment in a late stage of the manufacturing process requires a highly sophisticated technique and technical know-how.

Thus, the inventors have found that by transforming a metallic structure of the stainless steel to an α′ phase by cold rolling of 1 to 80% and subjecting the steel to low temperature heat treatment of 250 to 480° C., a diffusion concentration of supersaturated solid solution carbon into a γ phase of a few % in a volume ratio is obtained by using a strain energy accumulated in the α′ phase as a driving force, and the adjacent α′ phase can be reversely transformed into a γ_(T) phase with the γ phase as a nucleus. Furthermore, the inventors have found that carbides of Cr and Mo are finely precipitated in the α′ phase by the heat treatment, and therefore, due to the process strain induced transformation (TRIP) effect by dispersing the γ_(T) phase simultaneously with the further increase in strength, the carbide of 0.2% yield strength (YS) of 1400 N/mm² or more and elongation (EL) of 15% or more can be realized. Furthermore, the inventors have found that it is possible to achieve both 0.2% yield strength (YS) of 1550 N/mm² or more and elongation (EL) of 23% or more under suitable conditions within the scope of the present invention, it is possible to realize a characteristic in which the value of the YS-EL balance obtained by the following formula (1) exceeds 35,000.

YS-EL balance=YS×EL  (1)

α′ phase is strain induced martensite phase.

γ_(R) phase is retained austenite phase.

γ_(T) phase is reversely transformed austenite phase.

The present invention provides a steel strip or a steel sheet which has both high strength, high ductility, and high corrosion resistance, and a manufacturing method thereof.

Means to Solve Problem

According to an embodiment, a metastable austenite stainless steel strip or steel sheet includes: a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe and unavoidable impurities; a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ_(T) phase and a γ_(R) phase, a total of the γ_(T) phase and the γ_(R) phase is 15 to 50 volume %, and a γ_(T) phase area ratio defined by the following formula (2) is between 1% and 20% inclusive; and 0.2% yield strength (YS) of 1400 N/mm² to 1900 N/mm² where a value of YS-EL balance derived from the formula (1) satisfies at least 21,000 to 48,000.

According to an embodiment, a manufacturing method of a metastable austenite stainless steel strip or steel sheet includes the steps of: subjecting cold rolling to a stainless steel strip or steel sheet of the above composition to form a strain induced martensite phase (α′ phase) from an austenite phase (γ phase); and subjecting a low temperature heat treatment in a temperature of 250 to 480° C. to the stainless steel strip or steel sheet, thereby growing an austenite phase (γ_(T) phase) from the strain induced martensite phase (α′ phase).

Here, YS-EL balance=YS×EL  (1) and

γ_(T) phase area ratio (%)=100×(total area ratio of γ_(T) phase of entire observation area)  (2)

The α′ phase is a strain induced martensite phase, the γ phase is combined the γ_(T) phase and the γ_(R) phase, the γ_(R) phase is a reverse transformation austenite phase in which an area per crystal grain is between 5 μm² and 20 μm² inclusive, the γ_(R) phase is an austenite phase other than the γ_(T) phase, and YS is 0.2% yield strength and EL is elongation.

The structure including the above phases involves both characteristics of 0.2% yield strength (YS) of 1400 N/mm² or more and elongation (EL) of 15% or more. The inventors consider that the former characteristic is achieved by the α′ phase hardened by carbide precipitation of Cr and/or Mo and the latter characteristic is achieved from the TRIP effect of the γ_(T) phase diffused in the α′ phase.

Now, the metastable austenite stainless steel strip or steel sheet of the present invention will be explained.

(Steel Composition)

The stainless steel strip or steel sheet of the present invention is a metastable austenite stainless steel including a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0%.

C is added 0.05% or more to achieve a strength required for the strain induced transformation during cold rolling and the α′ phase after the transformation. However, addition of C above 0.15% stabilize the austenite phase and the strain induced transformation during the cold rolling is blocked and secondary workability for punching or the like is deteriorated. Thus, the upper limit of C is set to 0.15%.

Si is an element necessary in the steel manufacturing as a deoxidizer, and is added 0.05% or more. However, Si above 1% lowers ductility and toughness, and thus, the upper limit of Si is set to 1%.

Mn is an element which stabilize the austenite phase as with Ni, and if the large amount thereof is added, a structure having 50% or more strain induced α′ phase cannot be achieved through an ordinary cold rolling process. Thus, in the present application, the upper limit of Mn is set to 2%. The lower limit of Mn is not set specifically; however, 0.1% is preferable in order to deal with cracks during a hot rolling process.

Cr is added 16% or more in order to achieve the corrosion resistance as a stainless steel. However, 18% or more Cr stabilizes the austenite phase and a sufficient amount of strain induced transformation α′ phase cannot be achieved through an routinely carried out cold rolling process. Thus, in the present application, the upper limit of Cr is set to 18%.

Ni is an austenite stabilizing element and a certain amount thereof must be added to maintain the structure before the cold rolling in a metastable austenite state. In the present invention, the lower limit of Ni is 4% in order to achieve the metastable austenite phase after the solution treatment process. However, Ni above 11 stabilize the austenite phase and 50% or more volume ratio of strain induced transformation α′ phase cannot be achieved through an ordinary cold rolling. Thus, the upper limit of Ni is set to 11%.

Mo is a significant element of the present invention. Mo is known as an element effective to increase pitting corrosion resistance of stainless steels, and in the present invention, is also a precipitation strengthening element which is significant in the low temperature heat treatment. In the present invention, the lower limit of Mo carbide to achieve the precipitation strengthening of α′ phase is 2.5% and the upper limit of Mo is set to 3.5% since the amount of Mo exceeding the upper limit value saturates the precipitation strengthening property and the costs of alloy becomes disadvantage.

Furthermore, one or more elements such as Ti and/or Al may be added for precipitation strengthening. The amount of additional elements is approximately 0.1 to 3.5% and is set in consideration of a balance with the other elements. Furthermore, in order to increase the corrosion resistance of α′ phase after the strain induced transformation, Cu of 0.4 to 1.0% in mass % is preferably added. Cu below 0.4% does not show a remarkable corrosion resistance improvement effect, and Cu above 1.0% may cause a problem during the manufacturing process such as cracks in the hot rolling process.

As unavoidable impurities, P, N, S, and O may be contained in the steel strip and the steel sheet of the present invention. Such impurities do not block the purpose of the present invention as long as the amount thereof falls within a range acceptable in an ordinary manufacturing process.

(Metallic Structure)

The metastable austenite stainless steel strip or steel sheet includes: a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ_(T) phase and a γ_(R) phase, a sum of the γ_(T) phase and the γ_(R) phase is 15 to 50 volume % (α′ phase is 50 to 85 volume %), and a γ_(T) phase area ratio defined by the following formula (2) (=100×(total area ratio of γ_(T) phase of entire observation area)) is between 1% and 20% inclusive.

If the sum of the γ_(T) phase and the γ_(R) phase is below 15 volume % (α′ phase is above 85 volume %), the γ phase becomes insufficient, the TRIP effect is lost, and the elongation decreases.

If the sum of the γ_(T) phase and the γ_(R) phase is above 50 volume % (α′ phase is below 50 volume %), the γ phase becomes excessive, the TRIP effect is lost, and the strength decreases.

If the γ_(T) phase area ratio is below 1%, the γ phase becomes insufficient, the TRIP effect is lost, and the elongation decreases.

If the γ_(T) phase area ratio is above 50%, the γ phase becomes excessive, the TRIP effect is lost, and the strength decreases.

(Characteristics)

The metastable austenite stainless steel strip or steel sheet with the above composition and metal structure can exert the characteristics of 0.2% yield strength (YS) of 1400 N/mm² to 1900 N/mm², or preferably, 1550 N/mm² to 1900 N/mm² where a value of YS-EL balance (=YS×EL) satisfies at least 21,000 to 48,000, or preferably, range of 35,000 to 48,000.

(Manufacturing Method)

The metastable austenite stainless steel strip or steel sheet with the above metallic structure and characteristics can be manufactured by subjecting cold rolling to the stainless steel strip or steel sheet of the above composition to form a strain induced martensite phase (α′ phase) from an austenite phase (γ phase) and subjecting a low temperature heat treatment in a temperature of 250 to 480° C. to the stainless steel strip or steel sheet in which the strain induced martensite phase (α′ phase) is formed in order to grow an austenite phase (γ_(T) phase) from the martensite phase (α′ phase) formed in the strain induced martensite phase formation process.

The inventors consider that the above characteristics is achieved in the metastable austenite stainless steel strip or steel sheet through the following mechanism. A low temperature heat treatment is subjected in the metallic structure such that supersaturated solid solution carbon stored in the α′ phase is diffused and concentrated into a micro γ_(R) phase which is a core of the reverse transformation using a strain energy accumulated in the α′ phase which is transformed from the γ phase during the cold rolling through the strain induced transformation as a driving force and the γ phase grows. Furthermore, while being maintained in a certain temperature, the precipitation hardening phenomenon of the α′ phase proceeds. By controlling the above conditions with various parameters, the strength of the α′ phase and the high ductility of γ phase by the strain induced transformation can be achieved. That is, a value of the YS-EL balance derived from formula (1) satisfies 21,000 or more.

Note that if the ratio of the α′ phase after the cold rolling is less than 50%, the strain energy accumulated in the α′ phase is low and insufficient, and thus, the diffusion and concentration of C in the γ phase from the α′ phase do not occur. Thus, the cold rolling rate becomes low, and the dislocation density in the α′ phase becomes low. Thus, a balance between strength and elongation, that is, a value of the YS-EL balance does not exceed that of conventional materials.

YS-EL balance=YS×EL  (1)

(Volume Ratio)

Evaluation of the martensite phase (α′ phase) and the austenite phase (γ phase) of the present invention is performed through an electron backscatter diffraction (EBSD) method. In the EBSD, while the number of crystal grains included in the observation area is at least 1,000, an area of 0.05×0.05 mm with respect to a surface vertical to the rolling direction of the steel material (that is, an RD surface) is observed. An area ratio calculated from a measurement result of the phase where misorientation of 5° or more is defined as a grain boundary is converted into a volume ratio. The same applied to volume %.

(Characteristics)

The metastable austenite stainless steel strip or steel sheet with the above composition and metal structure can exert the characteristics of 0.2% yield strength (YS) of 1400 N/mm² and elongate (EL) of 15% or more. By satisfying these two characteristics, the value of YS-EL balance is at least 21,000. Furthermore, within a suitable condition of scope of the present invention, 0.2% yield strength (YS) of 1550 N/mm² or more and elongation (EL) of 23% or more can be achieved, and thus, a characteristics of the value of YS-EL balance of more than 35,000 can be achieved. The characteristics involve both strength and ductility higher than those of conventional stainless steel strip and steel sheet.

(Method)

The above metallic structure and characteristics of the present invention is achieved through the following manufacturing method, for example. The manufacturing method will be explained in comparison with a routinely carried out manufacturing method of stainless steel.

Initially, a routinely carried out manufacturing method of stainless steel strip or steel sheet will be explained. Then, the manufacturing method of the stainless steel strip or steel sheet of the present invention will be explained.

A conventional manufacturing method of a precipitation strengthening metastable austenite stainless steel strip (for example, SUS631(17-7PH)) includes rolling a skin pass-finished stainless steel strip obtained through an ordinary method with 85% rolling reduction ratio, for example, and a solid solution heat treatment. The solid solution heat treatment includes a solid solution treatment of the steel strip in a temperature of 1100° C. and water cooling. Then, a martensite transformation treatment is performed. Specifically, the steel strip is rolled with 60% rolling reduction ratio. Then, in order to use precipitation strengthening of intermetallic compounds, a precipitation hardening treatment is performed in a temperature of 475° C., for example. Through the above processes, a stainless steel strip having 0.2% yield strength (YS) of 1400 N/mm² while elongation (EL) is relatively low 1 to 10%. This is because the above processes are not to cause reverse transformation. Furthermore, if a reverse transformation treatment is performed in a temperature above a precipitation hardening treatment temperature, specifically, 500° C. or more, elongation (EL) may be increased but 0.2% yield strength (YS) is decreased. This is because not only the reverse transformation but also solution of precipitated intermetallic compounds into a mother phase are progressed. Therefore, through the above method, 0.2% yield strength (YS) of 1400 N/mm² cannot be achieved.

Now, an example of the manufacturing method of the stainless steel strip or steel sheet of the present invention will be explained.

First step: In the first step, a cold rolling is subjected to a stainless steel strip having the composition of the present invention (for example, SUS631(17-7PH)) obtained through an ordinary method. The cold rolling is intended to increase a ratio of α′ phase by the strain induced transformation. Thus, although the work ratio differs depending on the composition of the steel strip and the thickness of the steel sheet, the work ratio is set between 20 and 90%, or preferably, 30% or more.

Second step: In the second step, a solid solution heat treatment is subjected to the stainless steel strip after the rolling. The heat treatment is intended to perform the reverse transformation of the α′ phase after the strain induced transformation by the cold rolling into a γ_(T) phase such that C oversaturated in the α′ phase is dispersed evenly in the γ phase, and to uniform the metallic structure in a martensite transformation treatment performed next. Although a temperature of the solid solution heat treatment differs depending on the composition of the stainless steel strip, the temperature is set between 900° C. and 1150° C., and is preferably 1000° C. or more. Then, a rapid cooling (for example, water cooling) is performed after heating.

Third step: In the third step, a martensite transformation treatment is performed. Although a rolling reduction ratio (work ratio) in this treatment differs depending on the required characteristics, composition of steel strip, and thickness of steel sheet, etc., the ratio is set between 0% and 60% with respect to the steel material or steel strip before the treatment, or preferably between 5% and 40%.

When the rolling reduction ratio exceeds 60%, the γ phase serving as a nuclear of the reverse transformation becomes insufficient, and the structure of the scope of the present invention can not be obtained by the subsequent reverse transformation treatment.

Fourth step: In the fourth step, a low temperature heat treatment is subjected to the steel strip or steel sheet subjected to the martensite transformation treatment corresponding to the required characteristics in the third step where the treatment temperature is set between 250° C. and 480° C., or preferably between 300° C. and 450° C. If the temperature is below 250° C., the diffusion and concentration of the supersaturated solid solution carbon in the α′ phase do not occur sufficiently, and the γ phase does not grow. Thus, improvement of a balance of strength and ductility cannot be expected. Furthermore, the temperature above 480° C. is close to a temperature where the solid solution begins, and thus, the diffusion of the supersaturated solid solution carbon in the α′ phase is promoted, and stabilized γ phase grows excessively. Thus, the TRIP effect does not occur, and the ductility decreases together with the strength. In contrast, the steel strip or steel sheet manufactured through the first to fourth steps can exert an improved balance of strength (YS) and elongation (EL) with a change of the ratio of α′ phase to γ phase, and the characteristics of the present invention can be achieved.

Furthermore, in a PH stainless steel, when attempting to subject the reverse transformation heat treatment to the precipitation hardening temperature (for example, 500° C.) ordinary used for purpose of precipitating an intermetallic compound, an intermetallic compound precipitates. Thereby the strength (YS) is increased while ductility (EL) is greatly decreased. Thus, with respect to a PH stainless steel in which the intermetallic compounds are precipitated, a heat treatment is performed in a lower temperature (for example, 250 to 300° C.) as compared to the temperature used for a metastable austenite stainless steel. The inventors have found that a high strength and a high ductility can be achieved at the same time by using increase of γ_(T) phase by the low temperature heat treatment and carbide precipitation by low temperature heat treatment.

Furthermore, the inventors have found that when the precipitation hardening heat treatment is subjected at a temperature (for example, 500° C.) which is routinely carried out after forming into a desired shape, the diffusion of solute atoms is promoted to precipitate intermetallic compounds is accelerated, and further strength increase can be expected.

Considering the above, the inventors focused on PH stainless steels such as SUS631 described as a metastable austenite stainless steel strip or steel sheet excellent in balance between strength and ductility.

When the conditions from the first step to the fourth step are satisfied, the metastable austenite stainless steel strip or steel sheet in which a value of the YS-EL balance exceeds at least 21,000 can be manufactured.

According to the manufacturing method of the present invention, without largely deviating from the scope of the ordinary secondary work processing step and without significantly increasing the production costs and environmental load, it is possible to obtain a stainless steel strip or steel sheet simultaneously having two characteristics which can not be attained by the conventional method can be produced. Furthermore, according to the manufacturing method of the present invention, the manufacturing steps shown in the first step and the second step may be repeated depending on the state of the raw material and thereafter the martensite transformation treatment of the third step is performed.

Note that the above manufacturing method of the stainless steel strip or steel sheet is merely an example, and the present invention may be manufactured through other methods.

Advantages of Invention

According to the present invention, it is possible to achieve both high strength, which is a feature of metastable austenitic stainless steel, and ductility which is a feature of high formability steel sheet, at a high level.

The stainless steel strip or steel sheet of the present invention can be applied to a component which requires extremely high strength in terms of structure which can not be realized by a conventional high strength material and which enables design of components of more complicated shape.

The metastable austenite stainless steel strip which is used as a base largely contains Cr and Ni and has superior corrosion resistance to high strength and high ductility materials such as steel plates for automobile, and thus, an antirust surface treatment after the steel manufacturing process may be unnecessary. In this way, it can be expected to be applied not only to strength and ductility but also to applications of the technical field requiring corrosion resistance.

In the conventionally known metastable austenitic stainless steel strip, the 0.2% yield strength (YS) rises but the elongation (EL) decreases with an increase in the working ratio of cold rolling. Not only is this inferior in workability, but also in the material of precipitation hardening system, dimensional change due to heat treatment after processing is inevitable.

In contrast, the metastable austenitic stainless steel strip of the present invention not only has a high 0.2% proof stress (YS) exceeding 1400 N/mm² but also obtains elongation (EL) exceeding 15% at the same time.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 1 described in Table 2 below.

FIG. 2 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 2 described in Table 2 below.

FIG. 3 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 3 described in Table 2 below.

FIG. 4 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 4 described in Table 2 below.

FIG. 5 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 5 described in Table 2 below.

FIG. 6 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 6 described in Table 2 below.

FIG. 7 is a drawing substitution microscopic photograph showing a metallic structure image of a sample of Identification 7 described in Table 2 below.

FIG. 8 shows chronological changes of YS×EL values on the basis of temperatures of a low temperature heat treatment where a sample of steel 1 described in the Table 1 is used. Note that a dotted line indicates a case where a low temperature heat treatment time is 15 minutes, a solid line indicates a case where the time is 60 minutes, and a single dashed line indicates a case where the time is 360 minutes.

FIG. 9 shows changes of YS×EL values in temperatures on the basis of periods of a low temperature heat treatment where a sample of steel 1 described in the Table 1 is used. Note that a dotted line indicates a case where a temperature of a low temperature heat treatment is 300° C., a solid line indicates a case where the temperature is 400° C., and a single dashed line indicates a case where the temperature is 500° C.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, examples of the present invention will be explained. Note that the present invention is not limited to the examples.

Examples

Now, examples will be explained together with comparative examples.

Example steel 1 and comparative steels 2 to 4 with different amounts of Mo were prepared. Chemical compositions thereof are shown in Table 1. In the steel of Example 1 in Table 1, steels (samples of Identification 1 to 5) having the metallic structure within the scope of the present invention and steels having metallic structures outside the scope of the present invention (samples of Identification 6 and 7) were manufactured. The metallic structures of the steels are shown in Table 2. Furthermore, manufacturing conditions of the steels are shown in Table 3. As to each of the manufactured steels (samples of Identifications 1 to 7), hardness (HV), tensile strength (Ts), 0.2% yield strength (YS), and elongation (EL) were measured and shown in Table 4, respectively. Note that, in Tables 1 to 4, a mark * indicates that the value therewith is out of the scope of the present invention.

TABLE 1 (Composition) Chemical Composition (%) Steel C Si Mn P S Cr Ni Mo Remainder Example 0.1 0.2 0.72 0.018 0.027 16.68 4.26 2.8 N steel 1 0.1 Fe Bal Comparative 0.1 0.2 0.71 0.019 0.027 16.53 4.21 *2.2 N example 0.1 steel 2 Fe Bal Comparative 0.1 0.2 0.72 0.019 0.025 16.58 4.25 *1.7 N example 0.1 steel 3 Fe Bal Comparative 0.1 0.2 0.73 0.017 0.021 16.7 4.18 *3.8 N example  0.99 steel 4 Fe Bal *indicates value out of scope of present invention.

TABLE 2 (Structure) α′ γT γR Total γ Identification phase(%) phase(%)  phase(%) phase(%) 1 83.1% 1.9% 15.0% 16.9% 2 78.5% 3.0% 18.5% 21.5% 3 74.1% 3.8% 22.1% 25.9% 4 72.1% 14.8% 13.1% 27.9% 5 70.6% 15.3% 14.1% 29.4% 6 *49.6% *22.1% 28.3% *50.4% 7 *86.3% *0.9% 12.8% *12.9%  (total area of γ crystal grain groups having 5 to 20 μm² [μm²])/(observation area 40² [μm²]) *indicates value out of scope of present invention.

TABLE 3 (Step) Order of step 1 2 6 First Second 3 4 5 Low temperature 0 step step Third Fourth Fifth heat treatment Material Cold Heat step step step 3 thickness rolling treatment Cold Heat Cold Low temperature Identification Acceptance 1 2 rolling treatment rolling heat treatment 1 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 250° C. 2 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. 3 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. 4 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. 5 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. 6 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm *500° C.  7 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm *None 1: Temperature of cold rolling is within range of temperature of routinely carried out cold rolling, and cold rolling is performed below transformation point of various materials. 2: Heating time in heat treatment process is set using time reaching a predetermined temperature as reference based on characteristics of heat treatment equipment. 3: Heating time of low temperature treatment is set in order to obtain intended metallic structure and characteristics.

TABLE 4 (Characteristics) Ys-EL Ys EL HV Ts Identification index (N/mm²) (%) (3 kgf) (N/mm²) 1 23615 1423 16.6 458 1598 2 24242 1443 16.8 456 1576 3 24595 1415 18.3 470 1547 4 26455 1407 18.8 453 1545 5 31922 1432 22.3 472 1615 6 *18544.05 *1278.9 *14.5 426 1445 7 *14124.21 *1094.9 *12.9 450 1607 *indicates value out of scope of present invention.

As can be understood from the above results, the samples of Identifications 1 to 5 of Table 4 satisfied 0.2% yield strength (YS) exceeding 1400 N/mm² and the elongation (EL) where the γ phase exceeds 15%. In contrast, the samples of Identifications 6 and 7 as comparative examples did not satisfy 0.2% yield strength (YS) and elongation (EL) at the same time. FIGS. 1 to 7 show metallic structure images of these samples of Identifications 1 to 7, respectively.

Next, steel 1 having the composition within the scope of the present invention and steels 2 to 4 having the composition out of the scope of the present invention were prepared as in Table 1, and stainless steel strips were manufactured on the basis of various manufacturing conditions of Table 6. The metallic structures thereof are shown in Table 5, and their characteristics are shown in Table 7. In Tables 5 and 7, a mark * indicates that the value therewith is out of the scope of the present invention.

From the experimental results shown in Tables 5 to 7, the following points can be understood. That is, in the example steels, unless the low temperature heat treatment temperature exceeds 500° C., desired characteristics can be obtained regardless of the length of the heat treatment time. However, when the low temperature heat treatment temperature is 500° C., desired characteristics can not be achieved if the period of time becomes longer. Furthermore, the desired characteristics can not be achieved without performing the low temperature heat treatment.

On the other hand, in the comparative example steels, desired characteristics can not be obtained even if low temperature heat treatment is performed under appropriate temperature conditions.

TABLE 5 (Structure) α′ γ Identification phase(%) phase(%) Steel 1-a1 78.5% 21.5% Steel 1-a2 78.4% 21.6% Steel 1-a3 77.7% 22.3% Steel 1-a4 76.6% 23.4% Steel 1-a5 70.8% 29.2% Steel 1-a6 74.1% 25.9% Steel 1-b1 72.1% 27.9% Steel 1-b2 69.7% 30.3% Steel 1-b3 61.7% 38.3% Steel 1-b4 62.2% 37.8% Steel 1-b5 61.6% 38.4% Steel 1-b6 70.6% 29.4% Steel 1-b7 63.1% 36.9% Steel 1-c1 50.8% 49.2% Steel 1-c2 51.6% 48.4% Steel 1-c3 56.6% 43.4% Steel 1-c4 55.6% 44.4% Steel 1-c5 *49.5% *50.5% Steel 1-c6 *49.6% *50.4% Steel 1-00 *86.3% *13.7% Steel 2-b6 71.7% 28.3% Steel 3-b6 63.0% 37.0% Steel 4-b6 87.1% 12.9% Note 1: In steels 1 to 4, a, b, and c as suffix after hyphen (-) indicate temperatures of low temperature heat treatment of 300° C., 400° C., and 500° C., respectively. Note 2: In steels 1 to 4, 1, 2, 3, 4, 5, 6, and 7 as suffix after hyphen (-) indicate time of low temperature heat treatment of 1 minute, 15 minutes, 30 minutes, 60 minutes, 180 minutes, 360 minutes, and 780 minutes, respectively. Note 3: 00 indicates no low temperature heat treatment is performed. *indicates value out of scope of present invention.

TABLE 6 (Step) Order of steps 1 2 6 First Second 3 4 5 Low temperature 0 step step Third Fourth Fifth heat treatment Material Cold Heat step step step 3 thickness rolling treatment Cold Heat Cold Low temperature Identification Acceptance 1 2 rolling treatment rolling heat treatment Steel 1-a1 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. × 1 min  Steel 1-a2 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. × 15 min  Steel 1-a3 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. × 30 min  Steel 1-a4 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. × 60 min  Steel 1-a5 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. × 180 min Steel 1-a6 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 300° C. × 360 min Steel 1-b1 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 1 min  Steel 1-b2 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 15 min  Steel 1-b3 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 30 min  Steel 1-b4 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 60 min  Steel 1-b5 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 180 min Steel 1-b6 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 360 min Steel 1-b7 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 780 min Steel 1-c1 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 500° C. × 1 min  Steel 1-c2 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 500° C. × 15 min  Steel 1-c3 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 500° C. × 30 min  Steel 1-c4 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 500° C. × 60 min  Steel 1-c5 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 500° C. × 180 min Steel 1-c6 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 500° C. × 360 min Steel 1-00 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm — Steel 2-b6 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 360 min Steel 3-b6 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 360 min Steel 4-b6 1 mm 0.45 mm 1050~1150° C. 0.2 mm 1050~1150° C. 0.15 mm 400° C. × 360 min 1 Temperature of cold rolling is within range of temperature of routinely carried out cold rolling, and cold rolling is performed below transformation point of various materials. 2 Heating time in heat treatment process is set using time reaching a predetermined temperature as reference based on characteristics of heat treatment equipment. 3 Heating time of low temperature treatment is set in order to obtain intended metallic metal structure and characteristics.

TABLE 7 (Characteristics) Ys-EL Ys EL HV Ts Identification index (N/mm²) (%) (3 kgf) (N/mm²) Steel 1-a1 24242 1443 16.8 456 1576 Steel 1-a2 23835 *1369.8 17.4 472 1532 Steel 1-a3 22207 1433 15.5 472 1567 Steel 1-a4 27166 1570 17.3 466 1586 Steel 1-a5 25376 1484 17.1 462 1528 Steel 1-a6 24595 *1344 18.3 470 1547 Steel 1-b1 26455 1407 18.8 453 1545 Steel 1-b2 30720 1536 20.0 467 1565 Steel 1-b3 33170 1543 21.5 460 1587 Steel 1-b4 31620 1508 21.1 475 1567 Steel 1-b5 36787 1552 23.7 469 1582 Steel 1-b6 31922 1432 22.3 472 1615 Steel 1-b7 37085 1612 23.0 482 1637 Steel 1-c1 36015 1446 24.9 460 1508 Steel 1-c2 29903 1417 21.1 460 1510 Steel 1-c3 26801 *1381.5 19.4 459 1485 Steel 1-c4 27490 1417 19.4 457 1495 Steel 1-c5 *20148.43 *1236.1 16.3 434 1412 Steel 1-c6 *18544.05 *1278.9 *14.5 426 1445 Steel 1-00 *14124.21 *1094.9 *12.9 450 1607 Steel 2-b6 *20944 *1126 18.6 432 1386 Steel 3-b6 *14297 *1153 *12.4 397 1355 Steel 4-b6 *17109 1599 *10.7 488 1652 *indicates value out of scope of present invention.

FIG. 8 shows chronological changes of YS×EL values on the basis of a low temperature heat treatment temperature where the steps shown in Table 6 were performed using the sample of the example steel 1.

From FIG. 8, it can be seen that when the low temperature heat treatment temperature exceeds 480° C., especially when the low temperature heat treatment time becomes long, the intended YS×EL value can not be obtained. On the contrary, when the low temperature heat treatment temperature is less than 250° C., in particular, when the low temperature heat treatment time is short, it is understood that the intended YS×EL value can not be obtained. If the temperature of low temperature heat treatment is between 300° C. and 450° C., a desired YS×EL value can be achieved stably regardless of the period of time of treatment.

FIG. 9 is a graph showing the change in YS×EL value for each temperature according to the low temperature heat treatment time when the step shown in Table 6 was performed using the sample of the example steel 1.

It is understood from FIG. 9 that the YS×EL value is stable at a low level at a value of 22,000 or more at 300° C. and the YS×EL value at a high level at a value of 29,000 or more at 400° C. In contrast, at 500° C., the YS×EL value sharply decreases in the range of about 37,000 to 20,000 as the low-temperature heat treatment time becomes longer.

Thus, in a low temperature treatment, at a low temperature heat treatment temperature of 500° C. or more, there is a disadvantage that rapid deterioration of characteristics occurs due to the low temperature heat treatment time, resulting in quality instability.

INDUSTRIAL APPLICABILITY

The base of the present invention is a metastable austenite stainless steel including a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0%. 50% or more of strain induced martensite phase (α′ phase) as a parent phase is obtained by cold rolling, and the strain induced α′ phase obtained, preferably, through a low temperature treatment performed in a temperature between 250° C. and 480° C. and a γ phase (γ_(T) phase+γ_(R) phase) form a dual phase structure of the stainless steel strip or steel sheet where a γ_(T) phase area ratio defined by the following formula (2) is between 1% and 20% inclusive, and a remaining phase is a metal structure composed of α′ and γ_(R).

A method of performing the reverse transformation of a metal structure of a conventional steel in which Ni and Mn are 11% or less through a low temperature heat treatment performed in a temperature of 480° C. or less is a novel technique. Furthermore, the structure obtained from the above method can achieve 0.2% yield strength (YS) of 1400 N/mm² or more and exerted elongation (EL) of 15% or more in the γ phase.

The metastable austenite stainless steel strip which is used as a base largely contains Cr and Ni and has superior corrosion resistance to conventional iron-based high strength and high ductility materials. That is, the present invention can be expected to not only achieve high strength and ductility but also be used in the technical field where the corrosion resistance is required. Furthermore, if hardness is required, a stainless steel strip or steel sheet having the above characteristics and 450 or more Vickers hardness can be achieved.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A metastable austenite stainless steel strip or steel sheet comprising: a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe and unavoidable impurities; a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ_(T) phase and a γ_(R) phase, a total of the γ_(T) phase and the γ_(R) phase is 15 to 50 volume %, and a γ_(T) phase area ratio defined by the following formula (2) is between 1% and 20% inclusive; and 0.2% yield strength (YS) of 1400 N/mm² to 1900 N/mm² where a value of YS-EL balance derived from the formula (1) satisfies at least 21,000 to 48,000, wherein YS-EL balance=YS×EL  (1) and γ_(T) phase area ratio (%)=100×(total area ratio of γ_(T) phase of entire observation area)  (2), where the α′ phase is a strain induced martensite phase, the Y phase is a combination of the γ_(T) phase and the γ_(R) phase, the γ_(R) phase is a reverse transformation austenite phase in which an area per crystal grain is between 5 μm² and 20 μm² inclusive, the γ_(R) phase is an austenite phase other than the γ_(T) phase, and YS is 0.2% yield strength and EL is elongation.
 2. The stainless steel strip or steel sheet according to claim 1, wherein the 0.2% yield strength (YS) is 1550 N/mm² to 1900 N/mm² and the value of YS-EL balance derived from the formula (1) satisfies at least 35,000 to 48,000.
 3. The stainless steel strip or steel sheet according to claim 1, wherein, in the mass percent composition, Fe is partly replaced with one or two elements selected from a group of Al: 0.1 to 3.5% and Ti: 0.1 to 3.5%.
 4. The stainless steel strip or steel sheet according to claim 1, wherein HV is 450 or more.
 5. A manufacturing method of a metastable austenite stainless steel strip or steel sheet, the method comprising: preparing a stainless steel strip or steel sheet including a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe and unavoidable impurities; performing cold rolling to the stainless steel strip or steel sheet in order to form 50 volume % or more of a strain induced martensite phase (α′ phase) from the austenite phase (γ phase); and performing a low temperature heat treatment to the stainless steel strip or steel sheet with the strain induced martensite phase (α′ phase) within a temperature range of 250 to 480° C. in order to grow a austenite phase (γ_(T) phase) from the martensite phase (α′ phase) formed in the strain induced martensite phase formation, wherein the metastable austenite stainless steel strip or steel sheet has the following metal structure and mechanical property, a metal structure is a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ_(T) phase and a γ_(R) phase, a total of the γ_(T) phase and the γ_(R) phase is 15 to 50 volume %, and a γ_(T) phase area ratio defined by the following formula (2) is between 1% and 20% inclusive, a mechanical property satisfies 0.2% yield strength (YS) of 1400 N/mm² to 1900 N/mm² where a value of YS-EL balance derived from the formula (1) satisfies at least 21,000 to 48,000, YS-EL balance=YS×EL  (1) and γ_(T) phase area ratio (%)=100×(total area ratio of γ_(T) phase of entire observation area)  (2), where the α′ phase is a strain induced martensite phase, the Y phase is a combination of the γ_(T) phase and the γ_(R) phase, the γ_(R) phase is a reverse transformation austenite phase in which an area per crystal grain is between 5 μm² and 20 μm² inclusive, the γ_(R) phase is an austenite phase other than the γ_(T) phase, and YS is 0.2% yield strength and EL is elongation.
 6. The manufacturing method according to claim 5, wherein the 0.2% yield strength (YS) is 1550 N/mm² to 1900 N/mm² and the value of YS-EL balance derived from the formula (1) satisfies at least 35,000 to 48,000.
 7. The manufacturing method according to claim 7, wherein the stainless steel strip or steel sheet contains, in the mass percent composition, Fe is partly replaced with one or two elements selected from a group of Al: 0.1 to 3.5% and Ti: 0.1 to 3.5%.
 8. The manufacturing method according to claim 5, wherein the stainless steel strip or steel sheet has HV of 450 or more. 